There is a continuing demand for photographic films and papers that have a greater sensitivity to light while minimizing fog and graininess.
One of the principle ways of increasing the general sensitivity to light of a silver halide photographic emulsion is by the process of chemical sensitization in which a small quantity of certain types of chemical reagents are digested with the silver halide emulsion. Correspondingly, there is continuing need for new chemical reagents, known as chemical sensitizers, that will produce still higher sensitivity with silver halide emulsions. It is desired in particular to find more effective chemical sensitizers for silver halide emulsions that contain large size silver halide grains to satisfy the demand for high speed photographic films and papers.
One of the most important types of chemical sensitizers is the sulfur sensitizers. These compounds contain labile sulfur atoms that form silver sulfide on the silver halide emulsion grains during digestion. Examples of sulfur sensitizers include sodium thiosulfate and various thiourea compounds. A particular class of tetrasubstituted thiourea compounds containing a nucleophilic substituent group was disclosed in Burgmaier and Herz, U.S. Pat. No. 4,810,626. These compounds were shown to be very effective sensitizers under mild digestion conditions and were shown to produce higher speeds than many other thiourea compounds that were lacking the specified nucleophilic substituents. There was, however, no teaching that these sensitizers would produce higher speeds than the more common sulfur sensitizer, sodium thiosulfate, for silver halide emulsions in general, provided that the temperature and time duration for the thiosulfate digestion would be adjusted to the optimum. Subsequent to the disclosure by Burgmaier and Herz, these tetrasubstituted thiourea compounds with the specified nucleophilic substituents were disclosed as sensitizers for tabular grain emulsions comprising epitaxially deposited silver halide protrusions at the comers and edges of the host tabular emulsions (e.g., Daubendiek et al U.S. Pat. Nos. 5,576,168 and 5,573,902; Olm et al U.S. Pat. Nos. 5,503,970 and 5,576,171; and Deaton et al U.S. Pat. No. 5,582,965). But tabular emulsions not involving the epitaxial deposition of silver halide protrusions are generally preferred since fewer steps are required in manufacturing.
There have also been examples disclosed of non-epitaxy tabular grain emulsions that have been sensitized with the thiourea compounds of Burgmaier and Herz, but all of these examples were for tabular emulsions not greater than about 3.5 .mu.m area weighted mean equivalent circular diameter as determined by examination of the grains by electron microscopy (see, for example, Deaton, U.S. Pat. No. 5,049,485, Example 3). There was no evidence that any higher sensitivity could be obtained with the thiourea compounds than with sodium thiosulfate.